U.S. patent number 4,890,063 [Application Number 07/230,195] was granted by the patent office on 1989-12-26 for probe coil system for magnetic resonance apparatus.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Motoji Haragashira.
United States Patent |
4,890,063 |
Haragashira |
December 26, 1989 |
Probe coil system for magnetic resonance apparatus
Abstract
A probe coil system for a magnetic resonance (MR) apparatus
includes a plurality of coil elements, a plurality of shortening
capacitor elements, and first and second capacitors. The plurality
of shortening capacitor elements are inserted between and in series
with the coil elements. The first capacitor is connected in
parallel with the coil elements and the shortening capacitor
elements. The second capacitors are connected at least in series
with the coil elements and the shortening capacitor elements. Each
of the shortening capacitor elements includes a plurality of
capacitors and a plurality of switches, selectively opened and
closed to switch a total capacitance of the plurality of
capacitors, allowing selection of one of a plurality of resonance
frequencies by adjusting the first and second capacitors within a
predetermined range.
Inventors: |
Haragashira; Motoji (Tochigi,
JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Kawasaki, JP)
|
Family
ID: |
16410910 |
Appl.
No.: |
07/230,195 |
Filed: |
August 9, 1988 |
Foreign Application Priority Data
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Aug 10, 1987 [JP] |
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62-199622 |
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Current U.S.
Class: |
324/322; 334/55;
324/318 |
Current CPC
Class: |
G01R
33/3635 (20130101) |
Current International
Class: |
G01R
33/32 (20060101); G01R 33/36 (20060101); G01R
033/20 () |
Field of
Search: |
;324/313,318,322
;333/174,175,176 ;334/55 ;343/722,745,749,750 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0141383 |
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May 1985 |
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EP |
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0175129 |
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Mar 1986 |
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EP |
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0276510 |
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Aug 1988 |
|
EP |
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0031978 |
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Feb 1986 |
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JP |
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1264310 |
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Oct 1986 |
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SU |
|
Other References
Chen, et al., "A Study of R.F. Power Deposition in Imaging", Book
of Abstracts, vol. 2, Society of Magnetic Resonance in Medicine
(Fourth Annual Meeting, Aug. 1985), London, U.K., pp. 918-919.
.
Shen, et al., "R.F. Coil Design for NMR Imaging", Book of
Abstracts, vol. 2, Society of Magnetic Resonance in Medicine
(Fourth Annual Meeting, Aug. 1985), London, U.K., pp.
1117-1118..
|
Primary Examiner: Chapman; John
Assistant Examiner: O'Shea; Kevin D.
Attorney, Agent or Firm: Foley & Lardner, Schwartz,
Jeffery, Schwaab, Mack, Blumenthal & Evans
Claims
What is claimed is:
1. A probe coil system for a magnetic resonance (MR) apparatus
which includes a circuit including a coil, said MR apparatus
performing at least one application of a radio frequency magnetic
field for exciting MR in an object and detecting an MR signal
generated from said object by said MR, said probe coil system
comprising:
a plurality of coil elements constituting said coil;
a plurality of shortening capacitor elements inserted between and
in series with said coil elements;
a first capacitor including a variable capacitor connected at least
equivalently in parallel with a series circuit including said coil
elements and said shortening capacitor elements; and
a second capacitor including variable capacitors connected at least
equivalently in series with and at both ends of said series
circuit,
wherein each of said shortening capacitor elements comprises a
plurality of capacitors and a plurality of switches, selectively
opened and closed to switch a total capacitance of said plurality
of capacitors and to allow selection of one of a plurality of
resonance frequencies by adjusting said first and second capacitors
within a predetermined range.
2. A probe coil system according to claim 1, wherein said plurality
of capacitors are connected in series with each other and said
plurality of switches is connected in parallel with said plurality
of capacitors.
3. A probe coil system according to claim 1, wherein said plurality
of capacitors are connected in parallel with each other and said
plurality of switches is connected in series with said plurality of
capacitors.
4. A probe coil system according to claim 1, wherein said plurality
of shortening capacitor elements include at least one shortening
capacitor element having a first portion of said plurality of
capacitors connected in series with each other and some of said
plurality of switches connected in parallel with said first portion
of capacitors, and at least one shortening capacitor element having
a second portion of said plurality of capacitors connected in
parallel with each other and some of said plurality of switches
connected in series with said second portion of capacitors.
5. A probe coil system according to claim 1, wherein said switches
comprise a PIN diode switch.
6. A probe coil system according to claim 1, wherein said switches
comprises a relay.
7. A probe coil system according to claim 1, wherein said switches
are controlled in association with each other between said
plurality of shortening capacitor elements.
8. A probe coil system for a magnetic resonance (MR) apparatus,
which includes a circuit including a coil, said MR apparatus
performing at least one application of a radio frequency magnetic
field for exciting MR in an object and detecting an MR signal
generated from said object by said MR, said probe coil system
comprising:
(n-1) coil elements constituting said coil, said (n-1) coil
elements equivalently having a total inductance of L0;
n shortening capacitor elements inserted between and in series with
said coil elements, each of said shortening capacitor elements
Cs(i) having a capacitance Cs(i), where i=1, 2, . . . n and n is an
integer greater than or equal to 2;
a first capacitor including a variable capacitor of capacitance C1
connected at least equivalently in parallel with a series circuit
said coil elements and said shortening capacitor elements; and
a second capacitor including variable capacitors of capacitance C2
connected at least equivalently in series with and at both ends of
said series circuit,
wherein each of said shortening capacitor elements comprises a
plurality of capacitors and a plurality of switches selectively
opened and closed to switch a total capacitance Cs of said
plurality of capacitors causing
to satisfy
where .omega.0 is an initial value of an angular frequency and
.omega.1 is a resonance angular frequency to be selected, such that
one of a plurality of resonance frequencies is selected by
adjusting said first and second capacitors within a predetermined
range.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a probe coil system, used in a
magnetic resonance (MR) apparatus for obtaining anatomical or
qualitative information of an object utilizing an MR phenomenon,
for applying an electromagnetic wave signal for exciting an MR
phenomenon in an object and/or detecting an MR signal from the
object and, more particularly, to a probe coil system for an MR
apparatus capable of transmitting an RF excitation signal
corresponding to a plurality of types of resonance frequencies or
detecting an RF resonance signal.
2. Description of the Related Art
The MR phenomenon is a phenomenon in which an atomic nucleus placed
in a static magnetic field and having a spin or magnetic moment
resonantly absorbs only an electromagnetic wave having a
predetermined frequency. This atomic nucleus resonates at angular
frequency .omega.0 (.omega.0=2.pi..nu.0, .nu.0: Larmor frequency)
represented as follows:
where .gamma. is the specific gyromagnetic ratio of the specific
atomic nucleus and HO is the static magnetic field intensity.
In such a system for diagnosing a living organism utilizing the MR
phenomenon, the MR phenomenon is excited in an object, and an
electromagnetic wave of the resonance frequency is induced after
absorption of the resonance is received and processed, thereby
obtaining information of, e.g., a tomographic image of, the
object.
In this system, in principle, the MR phenomena can be excited in
and MR signals can be acquired from all portions of the object.
However, due to limitations of an apparatus and clinical demands
for a diagnosis image, actual conventional apparatuses utilize a
gradient magnetic field to perform excitation of MR and acquisition
of the MR signal for a specific portion, e.g., a specific slice, of
an object.
For example, as shown in FIG. 1, a conventional medical diagnostic
MR imaging apparatus comprises bed 1, static magnetic field coil 2,
gradient magnetic field generation coil 3, probe coil system 4,
static field power supply 6, X-, Y-, and Z-gradient power supplies
7, 8, and 9, transmitter 10, receiver 11, sequencer 12, and control
processor 13. Bed 1 includes movable board 1a on which object P is
placed. Static magnetic field coil 2 is driven by power supply 6
and generates a static magnetic field. Gradient magnetic field
generation coil 3 is driven by power supplies 7, 8, and 9 and
generates X-, Y-, and Z-gradient magnetic fields, respectively.
Probe coil system 4 comprises at least one coil including a
transmitting coil and a receiving coil or a transmitting/receiving
coil for both transmission and reception. System 4 is driven by
transmitter 10 and transmits a rotational magnetic field which is
an RF signal for exciting MR. The MR signal induced in the object
is detected by receiver 11 through system 4. Sequencer 12 drives
and controls power supplies 7, 8, and 9 and transmitter 10 in
accordance with a predetermined pulse sequence. Control processor
13 controls operations of bed 1 and sequencer 12 and processes the
MR signal detected by receiver 11. Processor 13 includes a display
and outputs a result of signal processing, e.g., displays the
result on the display.
This system is used as follows.
Object P is placed on board 1a of bed 1, and board 1a is moved so
that object P is located in a static magnetic field generated by
static magnetic field coil 2. Then, transmitter 10 is driven by
sequencer 12 in accordance with the predetermined sequence and
causes probe coil system 4 to transmit, e.g., a 90.degree. or
180.degree. pulse as a rotational magnetic field, i.e., an
excitation pulse for exciting MR. At the same time, power supplies
7, 8, and 9 are driven to cause gradient magnetic field generation
coil 3 to apply a gradient magnetic field to object P.
Upon application of the excitation pulse and the gradient magnetic
field, an MR phenomenon is generated in at least a predetermined
portion of object P, and an induced MR signal is detected by system
4. The MR signal is fetched by control processor 13 and subjected
to image processing such as image reconstruction processing. As a
result, imaging information such as a tomographic image is obtained
and displayed.
System 4 will be described below.
In order to obtain anatomical information of a living organism such
as a slice image and qualitative information such as a spectroscopy
using the above apparatus, a plurality of nuclear species are used
or the static magnetic field is varied (e.g., an apparatus using a
rampable magnet capable of enhancing and reducing the static
magnetic field intensity in a short time period is used for
spectroscopy). In this case, an RF signal of a resonance frequency
applied from system 4 to object P or detected from object P differs
in accordance with the type of atomic nucleus or with the static
magnetic field intensity even if the atomic nucleus is not
changed.
Examples are 1H, 21.3 MHz at 0.5 T, 42.6 MHz at 1 T, and 64 MHz at
5 T; 31p, 8.6 MHz at 0.5 T, 17.2 MHz at 1 T, and 25.8 MHz at 1.5 T;
and 13C, 5.4 MHz at 0.5 T, 10.7 MHz at 1 T, and 16.1 MHz at 1.5
T.
In this case, a tuning frequency of a conventional probe coil
system 4 is unconditionally determined in accordance with an
inductance of the coil. For this reason, in order to use a
plurality of nuclear species and to vary the static magnetic field,
the tuning frequency of system 4 must be variably controlled.
According to an abstract "R.F. Coil Design for NMR Imaging (J. F.
Shen and I. J. Lowe)" of the "Society of Magnetic Resonance in
Medicine (Fourth Annual Meeting, Aug. 19-23, 1985)", a tuning
frequency can be changed by inserting a shortening capacitor in a
circuit system including a coil.
A probe coil system in which a shortening capacitor is inserted
will be described below.
FIG. 2 is a circuit diagram showing coil L consisting of a
plurality of coil elements of the probe coil system. FIG. 3 shows a
circuit in which shortening capacitors each consisting of a
plurality of capacitance elements are inserted between a plurality
of coil elements of coil L similar to that shown in FIG. 2. In this
case, assuming that the self resonance frequency of the circuit
shown in FIG. 2 is fself, the self resonance frequency fself'
obtained when shortening capacitor Cs consisting of a plurality of
capacitor elements is inserted as shown in FIG. 3 is represented as
follows:
FIG. 4 shows an equivalent circuit of a probe coil system obtained
by connecting, in a circuit mainly consisting of the coil shown in
FIG. 2, tuning capacitor C1 in parallel with the coil and matching
capacitor C2 in series therewith.
In FIG. 4, reference symbol L0 dentes an inductance of the coil;
r0, an equivalent resistance caused by the coil itself and object P
inserted therein; and Z0, an output impedance of the probe coil
system which is set to coincide with a characteristic impedance of
a cable connected to the probe coil system. If a circuit including
the shortening capacitor shown in FIG. 3 is a main element of a
circuit system of this probe coil system, shortening capacitor Cs
is a capacitance connected in series with L0 and Z0 as indicated by
broken lines representing Cs.
In this manner, a tuning frequency can be changed by inserting
shortening capacitor Cs in a circuit system including the coil of
the probe coil system. However, when shortening capacitor Cs is
inserted, the equivalent resistance r0 and output impedance Z0 are
changed. Therefore, tuning capacitor C1 and matching capacitor C2
must be adjusted.
For this reason, if shortening capacitor Cs inserted in the coil
portion is a continuously variable capacitor and the capacitance of
shortening capacitor Cs is changed to switch the tuning frequency
to a plurality of different frequencies, the number of portions of
the system to be adjusted is significantly increased. Therefore,
the frequency cannot be practically tuned to a plurality of tuning
frequencies. For this reason, in practice, a circuit in which a
shortening capacitor is not inserted as shown in FIG. 2 and a
circuit in which a suitable shortening capacitor is inserted as
shown in FIG. 3 are independently used. That is, the tuning
frequency is fixed in the conventional apparatus.
The tuning frequency can be varied by switching a circuit in which
the shortening capacitor is not inserted and a circuit in which the
shortening capacitor is inserted. In this case, however, capacitors
C1 and C2, which constitute a matching circuit, can be used as they
are only when the shortening capacitor has a particular
capacitance, which is very rare in practice. As a result, the
arrangement is complicated, and only two tuning frequencies can be
set. Therefore, according to the conventional techniques, the MR
signals cannot be acquired using a plurality of nuclear species or
varying a static magnetic field.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a
probe coil system for an MR apparatus capable of easily being set
to a plurality of tuning frequencies.
The probe coil system for an MR apparatus according to the present
invention, including a circuit system including a coil and used in
an MR apparatus, for performing at least one application of an RF
magnetic field for exciting MR in an object and detection of an MR
signal generated from the object by the MR, comprises: a plurality
of coil elements constituting the coil; a plurality of shortening
capacitor elements inserted between and in series with the coil
elements; a first capacitor including a variable capacitor at least
equivalently connected in parallel with a series circuit including
the coil elements and shortening capacitor elements; and a second
capacitor including variable capacitors at least equivalently
connected in series with both ends of the series circuit the coil
elements and the shortening capacitors, wherein each of the
shortening capacitor elements comprises a plurality of capacitances
and a plurality of switches, selectively opened and closed to
switch a total capacitance of the plurality of capacitances, for
selecting one of a plurality of resonance frequencies by adjusting
the first and second capacitors within a predetermined range.
With the above arrangement, a total capacitance of the shortening
capacitor elements including the plurality of capacitors can be
properly varied by combining opened and closed states of the
plurality of switches, thereby setting a plurality of tuning
frequencies. In addition, since this function can be performed by
only adjusting a capacitance of a tuning capacitor within a
predetermined range, the number of portions of the system to be
adjusted is small.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing a conventional arrangement of an MR
imaging system;
FIG. 2 is a circuit diagram showing an arrangement of a coil
portion in a conventional probe coil system;
FIG. 3 is a circuit diagram showing another arrangement of the coil
portion in the conventional probe coil system;
FIG. 4 is a circuit diagram showing an equivalent circuit of the
conventional probe coil system;
FIG. 5 is a circuit diagram showing an arrangement of an embodiment
of a probe coil system for an MR apparatus according to the present
invention;
FIG. 6 is a circuit diagram showing an arrangement of a coil
element of the system shown in FIG. 5;
FIG. 7 is a circuit diagram showing another arrangement of the coil
element of the system shown in FIG. 5;
FIG. 8 is a circuit diagram showing an equivalent circuit of the
system shown in FIG. 5;
FIG. 9 is a circuit diagram showing an equivalent circuit obtained
by further equivalently converting the equivalent circuit shown in
FIG. 8;
FIGS. 10 and 11 are circuit diagrams for explaining the embodiment
shown in FIG. 5; and
FIG. 12 is a circuit diagram showing an arrangement of a probe coil
system according to another embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of a probe coil system for an MR apparatus according
to the present invention will be described below with reference to
the drawings.
FIG. 5 is a circuit diagram showing an arrangement of the probe
coil system, and FIGS. 6 and 7 are detailed circuit diagrams of a
shortening capacitor shown in FIG. 5.
In the probe coil system shown in FIG. 5, a plurality of shortening
capacitor elements Cs(i), i.e., Cs(1), Cs(2), . . . Cs(n)
(n.gtoreq.2) are inserted between and in series with a plurality of
coil elements constituting coil L. As shown in FIG. 6, in each
shortening capacitor Cs(i), a plurality of capacitors Ci, i.e., Cl,
C2, . . . Cm are connected in series with each other, and a
plurality of switches SWi, i.e., SW1, SW2, . . . SWm each including
a static switch using a PIN diode or a contact switch using a relay
or the like are connected in parallel with corresponding capacitors
Ci. Alternatively, instead of the arrangement shown in FIG. 6,
shortening capacitor Cs(i) is arranged such that capacitors Ci (C1,
C2, . . . Cm) and switches SWi (SW1, SW2, . . . SWm) are connected
in series with each other and these series circuits are connected
in parallel with each other as shown in FIG. 7.
The plurality of shortening capacitor elements Cs(i) (Cs(1), Cs(2),
. . . Cs(n)) shown in FIG. 5 are switched by control circuit
21.
If the total capacitance of Cs(i) is defined as Cs(i) (i=1, 2, . .
. n; n.gtoreq.2), the following condition is satisfied:
Control circuit 21 selectively opens and closes switches SWi (SW1,
SW2, . . . SWm) shown in FIG. 6 or 7 in accordance with a
predetermined switching pattern.
A function of the probe coil system having the above arrangement
will be described below.
FIG. 8 shows an equivalent circuit of the probe coil system in
which shortening capacitor Cs is inserted in a coil portion. The
circuit in FIG. 8 can be represented by an equivalent circuit shown
in FIG. 9 by equivalent conversion.
Since a coil normally has a high Q,
r0<<{.omega.L0-(1/.omega.Cs)} can be assumed, and the
following equations can be established: ##EQU1## where
.omega.=2.pi.f (f: resonance frequency).
In addition, since R<-Z0 is normally assumed, the following
equations can be established: ##EQU2##
In this case, the following equation (5) can be established by
substituting equation (1) into equation (4). ##EQU3## where
L0>1/.omega..sup.2 Cs.
An abstract entitled "A Study of R. F. Power Deposition in
Imagining", (C. N. Chen, V. J. Sank and D. I. Hoult) of "Society of
Magnetic Resonance in Medicine (Fourth Annual Meeting, Aug. 19-23,
1985)" describes how the following equation can be established
between power WB consumed by an object and MR frequency f:
If alternating magnetic field B1 is proportional to RF current I
(i.e., B1.varies.I), flip angle .theta. of spin is given by:
where .DELTA.t is the application time of an RF pulse, and an
equivalent series resistance of the coil itself is much lower than
that caused by an object when the object is loaded. Therefore, it
will be understood that the following equation can be established
between total equivalent series resistance r0 and frequency f when
the object is loaded in the probe coil system:
This is because magnetic field B1 generated by the probe coil
system does not depend on frequency f when RF current I flows
through the coil constituting the probe coil system. Since
WB.varies.I.sup.2 r0, the following equation can be established
from equations (6) and (7):
Assume that two frequencies .omega.0 and .omega.1
(.omega.0<.omega.1) are set in the probe coil system. When
.omega.0 is set in the circuit shown in FIG. 10 in which shortening
capacitor Cs is not inserted and .omega.1 is set in the circuit
shown in FIG. 11 in which shortening capacitor Cs is inserted, the
following approximation (9) can be established in accordance with
FIG. 9, and the following approximation (10) can be established by
substituting equation (8) into equation (5) in accordance with FIG.
10: ##EQU4##
In this case, assuming that C2=C2' in approximations (9) and (10),
the following equation can be obtained:
therefore,
The following approximation (12) can be obtained from equations
(1), (2), and (3): ##EQU5##
The following approximations can be obtained by substituting
and
into approximation (12): ##EQU6##
Results of the above calculations can be summarized as follows.
In order to change the resonance frequency, i.e., the angular
frequency from .omega.1 to .omega.1, shortening capacitor Cs is set
to satisfy:
in accordance with approximation (11). As a result, tuning
capacitor Cl' is obtained as:
in accordance with approximation (13), and matching capacitor C2'
is obtained as:
That is, C1 and C2 are changed within a comparatively small range
with respect to the change in angular frequency from .omega.0 to
.omega.1.
Therefore, when the capacitance of tuning capacitor C1 (C1') is
preset to be adjustable within a range corresponding to a desired
frequency range, portion A shown in FIG. 4 can be used in common
for different frequencies by only switching shortening capacitor
Cs.
In the probe coil system shown in FIG. 5, by setting the
capacitance of the tuning capacitor to be adjustable within a range
of C1' corresponding to a predetermined frequency range, the
capacitance of shortening capacitor Cs can be changed stepwise with
a combination of opened and closed states of switch Swi (i=1, 2, .
. . , m), and elements other than the tuning and matching
capacitors nned not be changed. That is, output impedance Z0 can be
adjusted to a predetermined characteristic impedance for a
plurality of frequencies.
FIG. 12 is a circuit diagram showing an arrangement of the probe
coil system according to another embodiment of the present
invention. In this embodiment shown in FIG. 12, switch Swi (i=1, 2,
. . . , m) in FIG. 5 is constituted by a PIn diode. Coupling
capacitor Cc satisfies Cc>>C1, C2, . . . Cm. Choke coils RFC
are connected to both ends of PIN diodes PD and the other ends of
coils RFC are connected to control circuit 21. Coil RFC at one end
of each diode PD is connected to diode driver DD in control circuit
21, and choke coil RFC at its other end is grounded.
With the arrangement shown in FIG. 12, even if a capacitance
between the terminals of diode PD is large, this capacitance is
equivalent to a slightly-increased capacitance of coupling
capacitor Cj (j=1, 2, . . . N) connected in parallel with diode PD,
thereby posing no problem. In addition, since an equivalent
resistance of PIN diode PD changes from 0.1 .OMEGA. to 10 to 20
K.OMEGA., diode PD functions as an ideal switch.
The present invention is not limited to the above embodiment but
can be variously modified without departing from the spirit and
scope of the present invention.
* * * * *